Method and apparatus for controlling transmissions in communications systems

ABSTRACT

In a bandwidth allocation protocol for a mobile communications network, mobile terminals report their bandwidth requirements to the network, while the network controls the amount of bandwidth that is used by the mobiles in reporting their bandwidth requirements. The mobiles indicate the total quantity of data awaiting transmission, the maximum delay time of the most urgent portion of the data and the maximum delay time of the least urgent portion. If a collision occurs between transmission by two mobiles, the mobiles wait for an interval controlled by the network before attempting another contention-based access transmission. The network periodically varies the contention-based access capacity available according to the observed usage level and/or collision rate in the previously allocated contention-based access capacity. The network analyses the forward traffic to individual mobiles and predicts the return bandwidth requirements which are likely to result from the forward traffic. The network stores associations between forward and return frequency channels, so that when a mobile receiving a forward frequency channel request return capacity, the network preferentially assigns return bandwidth to the mobile in one or more of the associated return channels.

The present invention relates to communications apparatus and methods,particularly but not exclusively for wireless communications,particularly but not exclusively via satellite.

A number of wireless communications systems have already been proposedto support shared access by many simultaneous communications sessions ofdifferent types. For example, the patent publication WO 98/25358discloses a mobile satellite communications system which supports thevariable bandwidth requirements of multiple simultaneous communicationssessions.

With this type of system, it is difficult to allocate bandwidth to meetthe varying requirements of multiple terminals or sessions, while usingthe overall bandwidth efficiently. The bandwidth allocation protocolsthemselves incur a significant signalling overhead, but the moreinformation that is exchanged in these protocols, the better the networkis able to adapt to constantly changing demands for bandwidth. Somebandwidth may be designated as being available for contention-basedaccess, which allows data and signalling to be transmitted by mobileswithout a bandwidth allocation specific to that mobile, butcontention-based access is very bandwidth-inefficient; if theprobability of collision is to be kept low, much more bandwidth needs tobe allocated than is likely to be actually used.

According to one aspect of the present invention, there is provided abandwidth allocation protocol in a mobile communications network inwhich mobiles report their bandwidth requirements to the network, whilethe network controls the amount of bandwidth that is used by the mobilesin reporting their bandwidth requirements. In this way, the network cancontrol the signalling overhead used by the bandwidth allocationprotocol, so as to make more bandwidth available for user data when achannel becomes congested. Alternatively, when the channel is notcongested, the network can allow the mobiles to report changes in theirbandwidth requirements more quickly, increasing the likelihood that thequality of service demands by active communications sessions on themobiles can be met.

According to another aspect of the present invention, there is provideda bandwidth allocation protocol in which mobiles indicate both thequantity of data awaiting transmission and the maximum delayrequirements for transmission of that data. Instead of indicatingindividually the delay requirements of each block of data awaitingtransmission, the mobiles indicate the total quantity of data awaitingtransmission, the maximum delay time of the most urgent portion of saiddata and the maximum delay time of the least urgent portion. Thisprovides enough information for the network to allocate the necessarybandwidth at the right time to meet the delay requirements of all of thedata, while reducing the amount of information needed to indicate thedelay requirements.

According to another aspect of the present invention, there is provideda contention-based access protocol for wireless mobile terminals, inwhich, if a collision occurs between transmission by two mobiles, themobiles wait for an interval controlled by the network before attemptinganother contention-based access transmission. In one example, thenetwork transmits an interval range signal to the mobiles, indicating arange for the interval for which the mobiles must wait beforeretransmitting, and the mobiles select an interval within the range;preferably, this selection is random or pseudo-random. This protocolallows the network to control the likelihood of collision incontention-based access, without necessarily having to allocate morebandwidth to contention-based access; instead, some of the mobiles maybe forced to wait longer before retrying.

A further refinement of this protocol involves the network specifying afurther increment by which the mobiles must increase the range of theinterval each time a subsequent attempt at transmitting the same burstfails. If there are repeated collisions, this indicates that there isnot enough contention-based capacity to meet the current demands of themobiles. According to this refinement, mobiles experiencing repeatedcollisions are automatically spread over an increasingly broader rangeof contention-based access capacity to increase the chance of the burstgetting through, while the interval range applied by mobiles waitingafter their first unsuccessful transmission is not affected.

According to another aspect of the present invention, there is provideda method of managing contention-based access capacity for mobileterminals in a wireless network, in which the network periodicallyvaries the contention-based access capacity available according to theobserved usage level and/or collision rate in the previously allocatedcontention-based access capacity. This adaptive allocation has theadvantage of allowing excess allocation of contention-based accesscapacity to be avoided, while keeping collision rates at an acceptablelevel.

According to another aspect of the present invention, there is provideda method of allocating return bandwidth to mobiles in a network, inwhich the network analyses the forward traffic to individual mobiles andpredicts the return bandwidth requirements which are likely to resultfrom the forward traffic. At least two possible analytical approachesmay be taken, separately or in combination: interpreting the forwardtraffic by identifying for example requests to send data or to set upspecific types of call, and forming a statistical model relatingpatterns of forward traffic to patterns of return traffic. This aspecthas the advantage that the mobile does not need to request additionalbandwidth because the network can detect that it is required andallocate it in advance, thus reducing the signalling overhead andreducing the delay before the required bandwidth becomes available.

According to another aspect of the present invention, there is provideda frequency channel allocation scheme in which a wireless network storesassociations between forward and return frequency channels, so that whena mobile receiving a forward frequency channel requests return capacity,the network preferentially assigns return bandwidth to the mobile in oneor more of the associated return channels. As a result, mobiles assignedcapacity in a particular set of return channels are likely to be tunedto a small number of different forward channels, so that bandwidthallocation schedules for return channels need only be transmitted on asmall number of associated forward channels.

Aspects of the present invention extend to apparatus adapted to carryout the above methods and protocols, as well as signals generated bythese methods and protocols.

Specific embodiments of the present invention will now be described withreference to the accompanying drawings, in which:

FIG. 1 is a diagram of components of a satellite communication systemincorporating embodiments of the present invention;

FIG. 2 shows the channels used for communication between the SAN and theMAN's in a packet data service implemented in the system of FIG. 1;

FIG. 3 is a diagram of transmitter and receiver channel units in asatellite access node (SAN) of the system of FIG. 1;

FIG. 4 is a diagram of transmitter and receiver channel units in aMobile Access Node (MAN) of the system of FIG. 1;

FIGS. 5 a to 5 d show the structure of one of the LESP channels of FIG.4;

FIG. 6 a shows the burst structure of a 5 ms burst in one of the MESPchannels of FIG. 4;

FIG. 6 b shows the burst structure of a 20 ms burst in one of the MESPchannels of FIG. 4;

FIG. 7 is a timing diagram illustrating the operation of an initialtiming correction protocol for correcting the timing of transmissions inthe MESP channels;

FIG. 8 a is a timing diagram illustrating the timing of a transmissionin one of the MESP channels immediately following a timing correction;

FIG. 8 b is a timing diagram illustrating the timing of a transmissionin one of the MESP channels at an interval after a timing correction,where there is timing uncertainty;

FIG. 9 is a diagram of a MAC layer in one of the MAN's; and

FIG. 10 is a diagram of a MAC layer in one of the SAN's.

System Overview

FIG. 1 shows the principal elements of a satellite communications systemin an embodiment of the present invention. A plurality of Mobile AccessNodes (MAN) 2 communicate via a satellite 4 with a satellite earthstation, hereinafter referred to as a Satellite Access Node (SAN) 6. Thesatellite 4 may for example be an Inmarsat-3™ satellite, as describedfor example in the article ‘Launch of a New Generation’ by J R Asker,TRANSAT, Issue 36, January 1996, pages 15 to 18, published by Inmarsat,the contents of which are included herein by reference. The satellite 4is geostationary and projects a plurality of spot beams SB (five spotbeams in the case of an Inmarsat-3™ satellite) and a global beam GB,which encompasses the coverage areas of the spot beams SB, on theearth's surface. The MAN's 2 may be portable satellite terminals havingmanually steerable antennas, of the type currently available for usewith the Inmarsat mini-M™ service but with modifications as describedhereafter. There may be a plurality of SAN's 6 within the coverage areaof each satellite 4 and capable of supporting communications with theMAN's 2 and there may also be further geostationary satellites 4 withcoverage areas which may or may not overlap that of the exemplarysatellite 4. Each SAN 6 may form part of an Inmarsat Land Earth Station(LES) and share RF antennas and modulation/demodulation equipment withconventional parts of the LES. Each SAN 6 provides an interface betweenthe communications link through the satellite 4 and one or moreterrestrial networks 8, so as to connect the MAN's 2 to terrestrialaccess nodes (TAN) 10, which are connectable directly or indirectlythrough further networks to any of a number of communications services,such as Internet, PSTN or ISDN-based services.

Channel Types

FIG. 2 shows the channels used for communication between a sample one ofthe MAN's 2 and the SAN 6. All communications under this packet dataservice from the MAN 2 to the SAN 6 are carried on one or more slots ofone or more TDMA channels, referred to as MESP channels (mobile earthstation—packet channels). Each MESP channel is divided into 40 msblocks, divisible into 20 ms blocks. Each 20 ms block carries either one20 ms burst or four 5 ms bursts, in a format which will be describedbelow.

All communications under this packet data service from the SAN 6 to theMAN 2 are carried on one or more slots of one or more TDM channels,referred to as LESP channels (land earth station—packet channels). Theslots are each 80 ms long, and comprise two subframes of equal length.

For the purposes of channel set-up and other network signalling, the MAN2 also communicates with a network co-ordination station (NCS) 5, as isknown in the Inmarsat Mini-M™ service. The SAN 6 communicates throughthe network 8 to a regional land earth station (RLES) 9 whichcommunicates with the NCS 5 so as to perform channel set-up and othernetwork signalling.

Satellite Link Interface

The satellite link interface between the MAN's 2 and the SAN 6 to whichthe MAN's 2 are connected will now be described. This interface can beconsidered as a series of communications layers: a physical layer, amedium access control (MAC) layer and a service connection layer.

SAN Channel Unit

FIG. 3 shows the functions within the SAN 6 of a transmitter channelunit ST, which performs the transmission of data packets over a singlefrequency channel of the satellite link, and a receiver channel unit SR,which performs the reception of data packets over a single frequencychannel of the satellite link. Preferably, the SAN 6 includes multipletransmitter channel units ST and receiver channel units SR so as to beable to provide communications services to a sufficient number of MAN's2.

A hardware adaptation layer (HAL) 10 provides an interface between thechannel units and higher level software, and controls the settings ofthe channel units. In the transmitter channel unit ST, the HAL 10outputs data bursts Td which are scrambled by a scrambler 12, the outputtiming of which is controlled by a frame timing function 14 which alsoprovides frame timing control signals to the other transmitter channelunits ST. The scrambled data bursts are then redundancy encoded by anencoder 16, by means for example of a turbo encoding algorithm asdescribed in PCT/GB97/03551.

The data and parity bits are output from the encoder 16 to a transmitsynchronising function 18 which outputs the data and parity bits as setsof four bits for modulation by a 16QAM modulator 20. Unique word (UW)symbols are also input to the modulator 20 according to a slot formatwhich is described below. The output timing of the encoder 16, transmitsynchroniser 18 and modulator 20 is controlled by the HAL 10, which alsoselects the frequency of the transmit channel by controlling a transmitfrequency synthesiser 22 to output an upconversion frequency signal.This frequency signal is combined with the output of the modulator 20 atan upconverter 24, the output of which is transmitted by an RF antenna(not shown) to the satellite 4.

In the receiver channel unit SR, a frequency channel is received by anRF antenna (not shown) and downconverted by mixing with a downconversionfrequency signal at a downconverter 26. The downconversion frequencysignal is generated by a reception frequency signal synthesiser 28, theoutput frequency of which is controlled by the HAL 10.

In order to demodulate the received bursts correctly, the timing ofreception of the bursts is predicted by a receive timing controller 29,which receives the frame timing control information from the frametiming function 14 and parameters of the satellite 4 from the HAL 10.These parameters define the position of the satellite 4 and of its beamsand allow the timing of arrival of data bursts from the MAN's 2 to theSAN 6 to be predicted. The propagation delay from the SAN 6 to thesatellite 4 varies cyclically over a 24 hour period as a result of theinclination of the satellite's orbit. This delay variation is similarfor all of the MAN's 2 and is therefore used to modify the referencetiming of the MESP channels, so that the timing of the individual MAN's2 does not need to be modified to compensate for variations in satelliteposition.

The predicted timing information is output to each of the receivechannel units SR. The received bursts are of either 5 ms or 20 msduration according to a scheme controlled by the SAN 6. The HAL 10provides information about the expected slot types to a slot controller32, which also receives information from the receive timing controller29.

FIG. 3 shows separate reception paths for 5 ms and 20 ms bursts;references to functions on each of these paths will be denoted by thesuffixes a and b respectively. The slot controller 32 selects whichreception path to use for each received burst according to the predictedlength of the burst. The burst is demodulated by a 16QAM demodulator 34a/34 b and the timing of the burst is acquired by a UW acquisition stage36 a/36 b. Once the start and end of the burst is determined, the burstis turbo-decoded by a decoder 38 a/38 b and descrambled by a descrambler40 a/40 b. The recovered 5 or 20 ms data burst is then received by theHAL 10.

MAN Channel Unit

FIG. 4 shows the functions within one of the MAN's 2 of a receiverchannel unit MR and a transmitter channel unit MT. The MAN 2 may haveonly one each of the receiver and transmitter channel unit, for reasonsof compactness and cost, but if increased bandwidth capacity isrequired, multiple receiver and transmitter channel units may beincorporated in the MAN 2.

In the receiver channel unit MR a signal is received by an antenna (notshown) and down-converted by a down-converter 42 which receives adown-conversion frequency signal from a receive frequency signalsynthesiser 44, the frequency of which is controlled by an MAN hardwareadaptation layer 46. The down-converted signal is demodulated by a 16QAMdemodulator 48 which outputs the parallel bit values of each symbol to aUW detection stage 50, where the timing of the received signal isdetected by identifying a unique word (UW) in the received signal. Thetiming information is sent to a frame and symbol timing unit 52 whichstores timing information and controls the timing of the later stages ofprocessing of the signal, as shown in FIG. 4. Once the block boundariesof the received data have been detected, the received blocks are turbodecoded by a decoder 54, descrambled by a descrambler 56 and output asreceived bursts to the HAL 46.

In the transmitter channel unit MT, data for bursts of 5 or 20 msduration are output from the HAL 46. Separate paths identified by thesuffixes a and b are shown in FIG. 4 for the 5 and 20 ms burstsrespectively. The data is scrambled by a scrambler 48 a/48 b and encodedby a turbo encoder 50 a/50 b. Unique Words (UW) are added as dictated bythe burst format at step 52 a/52 b and the resultant data stream ismapped onto the transmission signal set at step 54 a/54 b and filteredat step 56 a/56 b. The transmission timing is controlled at atransmission timing control step 58 a/58 b. At this step, the TDMA slotposition is controlled by a slot control step 60 according to adesignated slot position indicated by the HAL 46. A timing offset isoutput by the HAL 46 and is supplied to a timing adjustment step 62which adjusts the timing of the slot control step 60. This timing offsetis used to compensate for variations in propagation delay caused by therelative position of the MAN 2, the satellite 4 and the SAN 6 and iscontrolled by a signalling protocol, as will be described in greaterdetail below. The sets of data bits are output at a time determinedaccording to the slot timing and the timing adjustment to a 16QAMmodulator 64. The modulated symbols are upconverted by an upconverter 66to a transmission channel frequency determined by a frequency output bya transmission frequency synthesiser 68 controlled by the HAL 46. Theupconverted signal is transmitted to the satellite 4 by an antenna (notshown).

LESP Channel Format

FIG. 5 a shows the frame structure of one of the LESP channels. Eachframe LPF has a duration of 80 ms and has a header consisting of aconstant unique word UW which is the same for all frames. The uniqueword UW is used for frame acquisition, to resolve phase ambiguity of theoutput of the demodulator 48 and to synchronise the descrambler 56 andthe decoder 54.

FIG. 5 b shows the structure of each frame, which consists of the uniqueword UW of 40 symbols. followed by 88 blocks of 29 symbols each followedby a single pilot symbol PS, terminating in 8 symbols so as to make upthe total frame length to 2688 symbols, of which 2560 are data symbols.These data symbols are divided, as shown in FIG. 5 c, into two subframesSF1, SF2 each encoded separately by the encoder 16, each of 5120 bits,making 1280 symbols. The encoder 16 has a coding rate of 0.509375, sothat each subframe is encoded from an input block IB1, IB2 of 2608 bits,as shown in FIG. 5 d. This structure is summarised below in Table 1:TABLE 1 LESP Frame Format Modulation 16QAM Data Rate (kbit/s) 65.2Interface frame length (ms) 80 Interface Frame Size (bits) 5120 Subframelength (ms) 40 Input Bits per Subframe 2608 Coding Rate 0.509375 OutputBit per Subframe 5120 Output Symbol Per Subframe 1280 Frame Length (ms)80 Data Symbol per Frame 2560 Pilot Symbol Insertion Rate 1/(29 + 1)Pilot Symbols per Frame 88 UW symbols 40 Frame Size 2688 Symbol Rate(ksym/s) 33.6MESP Channel Format

The MESP channel structure is based on 40 ms blocks with a channeltiming referenced to the timing of the associated LESP channel asreceived by the MAN's 2. Each 40 ms block can be divided into two 20 msslots, each of which can be further divided into four 5 ms slots, andthe division of each block into slots is determined flexibly by higherlevel protocols. FIG. 6 a shows the format of a 5 ms burst, consistingof a pre-burst guard time G1 of 6 symbols, a preamble CW of 4 symbols,an initial unique word UW1 of 20 symbols, a data subframe of 112symbols, a final unique word UW2 of 20 symbols and a post-burst guardtime G2 of 6 symbols.

The preamble CW is not intended for synchronisation purposes byreceivers (for example, the demodulators 30 a, 30 b) but convenientlyprovides a constant power level signal to assist the automatic levelcontrol of a high-power amplifier (HPA, not shown) in the transmittingMAN 2. In one example, each of the symbols of the preamble CW has thevalue (0,1,0,0). In an alternative format, the preamble may consist ofless than 4 symbols and the symbol times not used by the preamble CW areadded to the pre-burst and post-burst guard times G1, G2. For example,the preamble CW may be omitted altogether and the pre-and post-burstguard times increased to 8 symbols each.

The unique words include only the symbols (1,1,1,1), which is mappedonto a phase of 45° at maximum amplitude, and (0,1,0,1), which is mappedonto a phase of 225° at maximum amplitude. Hence, the unique words areeffectively BPSK modulated, although the symbols are modulated by the16QAM modulator 64. Indicating the (1,1,1,1) symbol as (1) and the(0,1,0,1) symbol as (0), the initial unique word UW1 comprises thesequence 10101110011111100100, while the final unique word UW2 comprisesthe sequence of symbols 10111011010110000111.

The 5 ms burst is designed for carrying short signalling messages ordata messages; the structure is summarised below in Table 2: TABLE 2 5ms Burst Structure Modulation 16QAM Input Bits per Burst 192 Coding rate3/7 Output Bits per Burst 448 Output Symbols per Subframe 112 Preamble 4Initial UW (symbols) 20 Final UW (symbols) 20 Total symbols 152 TotalGuard Time (symbols) 12 Symbol Rate (ksym/s) 33.6 Slot Length (ms) 5

FIG. 6 b shows the structure of a 20 ms burst of the MESP channel. Thesame reference numerals will be used to denote the parts of thestructure corresponding to those of the 5 ms burst. The structureconsists of a pre-burst guard time G1 of 6 symbols, a preamble CW of 4symbols, an initial unique word UW1 of 40 symbols, a data subframe of596 symbols, a final unique word of 20 symbols and a post-burst guardtime G2 of 6 symbols. The structure is summarised below in Table 3:TABLE 3 20 ms Burst Structure Modulation 16QAM Input Bits per Burst 1192Coding rate 1/2 Output Bits per Burst 2384 Output Symbols per Subframe596 Preamble 4 Initial UW (symbols) 40 Final UW (symbols) 20 Totalsymbols 660 Total Guard Time (symbols) 12 Symbol Rate (ksym/s) 33.6 SlotLength (ms) 20

The preamble CW has the same form and purpose as that of the 5 ms burst.The initial unique word UW1 comprises the sequence:0000010011010100111000010001111100101101while the final unique word UW2 comprises the sequence11101110000011010010, using the same convention as that of the 5 msburst.MESP Timing Correction

As shown above, the MESP slot structure incorporates a very short guardtime of about 0.24 ms at each end. However, the difference in the SAN 6to MAN 2 propagation delay between the MAN 2 being at the sub-satellitepoint and at the edge of coverage is about 40 ms for a geostationarysatellite, so the position of each MAN 2 will affect the timing ofreception of transmitted bursts in the MESP channel, and may causeinterference between bursts from MAN's 2 at different distances from thesub-satellite point. Moreover the satellite, although nominallygeostationary, is subject to perturbations which introduce a smallinclination to the orbit and cause the distance between the satellite 4and the SAN 6, and between the satellite 4 and the MAN 2, to oscillate.Although the position of the SAN 6 is fixed and that of the satellite 4can be predicted, the MAN's are mobile and therefore their positionschange unpredictably, and their clocks are subject to jitter and drift.

A timing correction protocol is used by the SAN 6 to measure thepropagation delay from the MAN 2 and send a timing correction value tothe MAN 2 to compensate for differences in propagation delay between thedifferent MAN's 2, so as to avoid interference between bursts fromdifferent MAN's caused by misalignment with the slots. The protocol willnow be illustrated with reference to the timing diagram of FIG. 7.

FIG. 7 shows LESP frames LPF including subframes SF1, SF2 and initialunique words UW. When the MAN 2 is switched on, or is able to acquireone of the LESP channels after an interval of not being able to do so,the MAN 2 receives (step 70) a 40 ms LESP subframe SF including returnschedule information which dictates the slot usage of a correspondingMESP channel. Return schedule information is transmitted periodicallywith a periodicity controlled by the SAN 6. The subframe SF includes thedesignation of a block of at least nine contiguous 5 ms slots as atiming acquisition group consisting of random access slots not assignedto any specific MAN 2. The MESP return schedule to which the subframe SFrelates begins 120 ms after the beginning of reception of the subframeSF. This 120 ms period allows 90 ms for the MAN 2 to demodulate the LESPsubframe SF (step 72) and 30 ms for the MAN 2 to initialise itself fortransmission (step 74).

At the beginning of the MESP return schedule there is allocated a timingallocation group of 5 ms slots. Initially, it is assumed that the MAN 2has the maximum timing uncertainty of 40 ms, corresponding to eight 5 msslots. Therefore, the MAN 2 can only transmit after the first eightslots of the timing acquisition group, and cannot transmit at all inacquisition groups containing less than nine slots, so as to avoidinterfering with transmissions in slots preceding the timing acquisitiongroup.

The MAN 2 randomly selects (step 78) one of the slots of the timingacquisition group following the first eight slots and transmits (step79) a burst in the selected slot, the burst including an indication ofthe slot selected. In the example shown in FIG. 7, the slots of thetiming acquisition group are numbered from 0 to M-1, where M is thenumber of slots in the timing acquisition group, and the number R,selected at random from 8 to M-1, is transmitted in the burst at step79. The burst may also indicate the type of the mobile, such asland-based, maritime or aeronautical.

The SAN 6 receives and records the time of arrival of the bursttransmitted by the MAN 2. From the slot number R indicated in the burst,the SAN 6 calculates the differential propagation delay to that MAN 2.Since the timing of transmission of the burst was (120+R×5) ms after thetime of reception of the LESP subframe SF, the timing of reception TR ofthe burst is approximately (2×DP+C+120+5×R) ms after the time oftransmission of the LESP subframe LPSF, where DP is the differentialpropagation delay to that MAN 2 and C is a delay which is the same forall the MAN's in a group, and includes various factors such as thepropagation delay to and from the satellite 4 and the retransmissiondelay of the satellite 4. Hence, in this example, the differentialpropagation delay is calculated as:DP=T _(R) −C−120−5×R  (1)

The SAN 6 then transmits to the MAN 2 a data packet indicating a timingcorrection offset X in the range 0 to 40 ms. The offset replaces theinitial timing offset of 40 ms in step 76, for subsequent transmissions.The MAN 2 receives the timing correction offset and adjusts itstransmission timing accordingly.

If the burst transmitted by the MAN 2 interferes with a bursttransmitted by another MAN 2 also attempting to receive a timingcorrection, the SAN 6 may not be able to read the contents of eitherburst and in that case will not transmit a timing offset correction toeither MAN 2. If the MAN 2 does not receive a timing offset correctionfrom the SAN 6 within a predetermined time, the MAN 2 waits for a randominterval within a predetermined range before attempting to transmit aburst in the next subsequently available timing acquisition group. Thepredetermined range of intervals is determined by a signalling packettransmitted by the SAN 6 which indicates maximum and minimum intervalsto be observed by MAN's 2 after a first unsuccessful transmission beforeattempting retransmission, together with a further waiting interval tobe added to the total waiting interval each time a furtherretransmission is made following an unsuccessful transmission.

FIG. 8 a illustrates the transmission timing of one of the MAN's 2 whichhas previously received a timing correction offset value X. As in FIG.7, the MAN 2 receives (step 80) the LESP subframe SF which includesreturn schedule information. The MAN 2 demodulates (step 82) the LESPsubframe LPSF and initialises (step 84) its transmitting channel unit,during a total allotted time of 120 ms after the beginning of receptionof the LESP subframe LPSF. The MAN 2 calculates the start of the MESPreturn schedule as being (120+X) ms from the beginning of reception ofthe subframe SF which carries the return schedule information. The MAN 2therefore waits for the timing offset period X (step 86) after the endof the 120 ms period before being able to transmit.

In this example, the return schedule dictated by the LESP subframe LPSFincludes a four 5 ms slots, followed by a 20 ms slot. If the MAN 2 hasbeen allocated a 20 ms slot, then it will transmit (step 88) in thedesignated 20 ms slot; if the MAN 2 has been allocated a 5 ms slot, thenit will transmit in the designated 5 ms slot. Alternatively, if the 5 msslots are designated as being random access slots and the MAN 2 has ashort packet that is due to be sent to the SAN 6, the MAN 2 selects oneof the four slots at random and transmits in that slot (step 89).

If the SAN 6 detects from the transmission by the MAN 2 that acorrection in the timing offset is needed, for example if the timebetween the start of the burst and the slot boundary as measured by theSAN 6 is less than a predetermined number of symbols, the SAN 6indicates a new timing correction to the MAN 2 in a subsequent datapacket. This may be indicated as an absolute timing offset X or as arelative timing offset to be added or subtracted from the current valueof X.

Timing Uncertainty

In the timing correction offset burst the SAN 6 transmits to the MAN 2,together with the timing offset, a timing uncertainty rate R_(U)indicating the rate at which the timing of the MAN 2 is likely tochange. For example, the timing uncertainty rate may represent a numberof symbols per second by which the MAN 2 is likely to change its timing.The SAN 6 determines the timing uncertainty rate from the class of theMAN 2 (e.g. land mobile, aeronautical) and other factors such as theinclination of the orbit of the satellite 6.

The MAN 2 times the interval elapsed since the last timing correctionwas received and multiplies this by the timing uncertainty rate R_(U) togive a timing uncertainty t_(U), wheret _(U)=MIN (T−T _(C) ×R _(U), 40 ms)  (2)where T is the current time and T_(C) is the time at which the lastcorrection was received. The MIN function means that the timinguncertainty cannot exceed the maximum uncertainty of 40 ms.

The timing offset X is reduced by the timing uncertainty t_(U) suchthat:X=MIN(X _(C) −t _(U), 0)  (3)where X_(c) is the initial value of X indicated in the last timingcorrection, the MIN function ensuring that X cannot fall below zero.

FIG. 8 b illustrates the transmission timing of one of the MAN's 2 withtiming uncertainty. Steps 80 to 84 correspond to those shown in FIG. 8 aand their description will not be repeated. At step 86, the MAN 2calculates the MESP return schedule as starting (120+X) ms after thebeginning of reception of the subframe SF, using the value of X asreduced by the timing uncertainty tu. As a result of the timinguncertainty t_(U), the MAN 2 must ignore the first I slots of a randomaccess group, whereI=INT[(t _(S) −t _(G) +t _(U))/t _(S)]  (4)t_(S) is the slot duration of 5 ms and t_(G) is the guard time G1, whichis 6 symbol periods in this case.

In the example shown in FIG. 8 b, there are four 5 ms slots at the startof the MESP return schedule, but tu is 7 ms, so that the first two slotsmust be ignored. The MAN 2 can then only transmit in the third andfourth slots.

If the timing uncertainty tu is greater than a predetermined value, suchas the value of the guard time. the MAN 2 reverts to the random accesstiming correction request process shown in FIG. 7 and inhibitstransmission in time slots allocated exclusively to itself, except wherea sufficient number of these are concatenated so that their total lengthcan accommodate both the timing uncertainty and the burst itself, untila new timing correction offset has been received from the SAN 6.However, the protocol differs from that of FIG. 7 in that the MAN 2 usesits current timing offset X instead of returning to the default value of40 ms in step 76. This protocol reduces the chance of interferencebetween bursts in allocated slots.

In the above embodiment, the timing offset X is reduced by the timinguncertainty t_(U) for all transmissions by the MAN 2. In an alternativeembodiment, the timing offset X is reduced by the timing uncertaintyt_(U) only for transmissions by the MAN 2 in random access slots, whilethe original timing offset X_(C) received in the last timing correctionmessage from the SAN 6 is applied when transmitting in allocated slots.In this alternative embodiment, it is important to distinguish betweentiming correction messages initiated by the SAN 6, after detection of atransmission by the MAN 2 in an allocated slot too close to the slotboundary, and timing correction messages sent by the SAN 6 in responseto a timing correction request by the MAN 2, which will have a differenttiming offset from the transmissions in allocated slots. Therefore, theSAN 6 indicates in the timing correction message whether this is beingsent in response to a request by the MAN 2, or was initiated by the SAN6. The MAN 2 then determines the new timing offset X_(C) from the timingoffset indicated in the timing correction message according to how thetiming correction message was initiated.

MAC Layer

As described above, the satellite link interface at each of the MAN's 2and at the SAN 6 includes a medium access control (MAC) layer whichprovides an interface between the physical layer, aspects of which aredescribed above, and the service connection layer, which provides accessto the satellite link for one or more service connections. The MAC layermay have a structure substantially as described in UK patent applicationno. 9822145.0. FIG. 9 illustrates the layer structure at the MAN 2, witha physical layer MPL managing the transmission of packets on one of theMESP channels and the reception of packets on one of the LESP channels,and the MAC layer MMAC dynamically mapping service connections at theservice connection layer MSCL to slots in the MESP and LESP channels.FIG. 10 illustrates the layer structure at the SAN 6, with a physicallayer LPL managing the transmission of packets on multiple LESP channelsand reception of packets on multiple MESP channels, and the MAC layerLMAC dynamically mapping service connections at the service connectionlayer LSCL to slots in the MESP and LESP channels.

The SAN MAC layer LMAC is responsible for allocating channel resourcesboth on the LESP and on the MESP channels. The MAN MAC layer MMACgenerates signalling packets indicating its current channel requirementsfor supporting the quality of service (QoS) requirements of all of theservice connections of the service connection layer MSCL. The term‘quality of service’ (QoS) includes one or more of minimum and maximumbitrate. average bitrate, and maximum delay requirements and may alsoinclude other requirements peculiar to certain types of communication.For example, where encryption is handled at the physical layer andencrypted data are transmitted on a dedicated channel, the quality ofservice may include an encryption requirement. The service connectionsmay specify, both when being set up and during the lifetime of a serviceconnection. QoS parameters without the need to specify how this QoS isto be achieved and it is the task of the MAC layer to meet the QoSrequirements of all its service connections in the mapping of theservice connections onto the physical layer. The MAN MAC layer MMACrequests the channel capacity necessary for this task by sendingsignalling packets to the SAN MAC layer LMAC.

The SAN MAC layer determines how the LESP channel slots are to beassigned to its own transmitting service connections, determines thesequence of 5 ms and 20 ms slots in each MESP channel and the allocationof these slots to the MAN's 2 or to random access, and transmitssignalling packets, indicating the slot sequences and allocations, inthe LESP channels. Each LESP subframe contains one-or more packets ofvariable length with any unused bits being filled with padding bits. TheMAN MAC layer MMAC receives the packet indicating its current allocationand decides how this allocation is to be divided between its serviceconnections.

Each MAC layer MAC receives data from service connections, formats thedata into packets, and maps the data packets onto physical channelsaccording to the current allocation scheme. Each data packet includes anidentifier field identifying to which service connection the packetbelongs. The receiving MAC layer receives data packets read by thephysical layer and assigns the data contents to the service connectionsidentified by the packets. The packets are of variable length dependingon their type and content, and each LESP subframe or MESP 5 or 20msburst can contain an integral number of packets, with padding if not allof the data bits are used.

Resource Management

Resource management algorithms are performed by the SAN MAC layer LMACin order to meet the QoS requirements of each MAN MAC layer MMAC asclosely as possible, as will now be described.

Periodically, the SAN 6 transmits a return schedule signalling packet onone or more of the LESP channels, indicating the allocation of slots inone of the MESP channels. The SAN MAC layer LMAC selects on which LESPchannel to transmit a return schedule signalling packet according to thecurrent allocation of MAN's 2 to the LESP channels and the MAN's whichare allocated capacity in the return schedule. Thus, a return schedulesignalling packet allocating MESP capacity to one of the MAN's 2 istransmitted on the LESP channel to which that MAN 2 is tuned. Tominimise the number of different return schedules which need to betransmitted, the SAN MAC layer LMAC stores an association table linkinga set of one or more MESP frequency channels to each of the LESPfrequency channels. Where a MAN 2 is tuned to a specified LESP channel,the SAN MAC layer LMAC preferentially assigns capacity to that MAN 2 onthe MESP channel or channels linked to that LESP channel. Theassociation table is not fixed, but may be modified by the SAN MAC layerLMAC. Each MESP channel may be associated with more than one LESPchannel.

The return schedule also allocates random access slots in the MESPchannels linked to the LESP channel on which the return schedule isbroadcast. Even if the whole of an MESP channel is allocated as randomaccess, the return schedule indicating this will be transmitted on eachof the forward bearers linked to that MESP channel.

Each MAN MAC layer MMAC sends signalling packets to the SAN MAC layerLMAC, including a queue status report indicating how much data needs tobe transmitted and the time at which the data needs to be sent. Thequeue status report has three fields: the latest delivery time of thedata packet at the head of the queue and therefore with highestpriority, the latest delivery time of the data packet at the tail of thequeue and therefore having the lowest priority, and the total length ofdata in the queue, as shown in Table 4 below: TABLE 4 Status PacketFormat Bits 8 7 6 5 4 3 2 1 Octet 1 x 0 0 0 1 1 0 0 Octet 2 <SeqNum> U <Octet 3 Queue Length> Octet 4 <Time Head Octet 5 > < Octet 6 Time Tail>where the fields are defined as follows:

SeqNum: Identifies the sequence number of the status packet, so that theSAN 6 can identify the sequence order of different status packets fromthe same MAN 2;

U: Small units flag, which identifies whether the subsequent queuelength is expressed in large or small units of data; the large units maybe equal to the capacity of a 20 ms slot;

Queue Length: the length of the data queue at the MAN2, expressed inlarge or small units according to the small units flag;

Time-Head: the delivery time, as an offset from the time of transmissionof the queue status report, of the first packet in the data queue; and

Time-Tail: the delivery time, as an offset from the time of transmissionof the queue status report, of the last packet in the data queue.

This format is particularly efficient in that it avoids transmitting thetransmission time requirements of each of the data packets, which wouldrequire too great a signalling overhead, while providing the SAN MAClayer LMAC with enough information to decide how much capacity, andwhen, to allocate to the requesting MAN 2.

However, the queue status reports still take up significant bandwidth onthe MESP channels which may be required to transmit data packets attimes of high loading. Moreover, the MAN MAC layer MMAC may transmitqueue status information in a contention based slot if no reservedcapacity is available, increasing the probability of collision in thecontention based slots. To reduce the contention slot loading, andtherefore to allow some of this bandwidth to be reclaimed for datapacket allocation, the SAN 6 may transmit reporting level controlsignalling packets addressed to all the MAN's 2. The control signallingpackets may indicate the minimum delay required before queue statusshould be reported in a contention slot, and also a reporting controlparameter which determines whether the MAN's 2 will transmit queuestatus information as soon as possible (subject to the minimum delay),as late as possible, or at a specified point between these two extremes.The latest possible delay is determined from the QoS delay requirementsand the round trip (MAN-SAN-MAN) delay and allows for only a minimumtime for the SAN 6 to allocate the return capacity on receipt of thequeue status information. Each MAN MAC layer MMAC, on receiving areporting level control signalling packet, applies the parametersindicated therein. In cases where the QoS demands of the serviceconnections to a MAN 2 increase very quickly, a long minimum reportinginterval and/or a high reporting control parameter may delay the MAN MAClayer's requests for capacity so that the SAN 6 is unable to meet therequired delay times indicated for all the MAN's within the QoS delayrequirements. A short minimum reporting interval and/or a low reportingcontrol parameter will increase the probability of the MAN MAC layerrequests reaching the SAN 6 in time for the required capacity to beallocated but will increase the number of contention slots required. TheSAN 6 may determine the appropriate parameters for the mix of trafficbeing carried.

The SAN MAC layer LMAC periodically allocates a contiguous block of atleast nine 5 ms slots as a timing acquisition group and transmits asignalling packet indicating this allocation. The length and frequencyof timing acquisition groups is allocated by the SAN MAC layer LMACaccording to anticipated demand (which may be determined by detectedtiming acquisition group loading), subject to a predetermined maximuminterval between timing acquisition groups, to allow efficient operationof the timing acquisition protocol.

The SAN MAC layer LMAC also determines the minimum and maximumrandomising intervals and further interval by which the MAN's 2 wait, asdescribed above, before retransmitting a timing acquisition burstfollowing an unsuccessful timing acquisition. These intervals determinethe timing spread of timing acquisition burst retransmissions and areselected so as to keep the probability of collision betweenretransmissions low, without causing excessive delay to the MAN's 2performing timing acquisition.

The SAN MAC layer LMAC also monitors the traffic transmitted on the LESPchannels in order to predict the future transmission capacity needs ofeach of the MAN's 2. For example, for each service connection which isoperating in ARQ mode, resources are allocated to the MAN 2 throughwhich the connection is operating when an ARQ time-out period is aboutto expire. Service-specific resource prediction may also be performed.For example, if the SAN MAC layer LMAC detects that a packet transmittedto a MAN 2 contains a request for transmission of a block of data, thecapacity necessary to transmit that block of data is allocated to theMAN 2 without waiting for the MAN 2 to request the additional capacity.However, it may not be possible to interpret the data contents ofpackets, for example if the contents are already encrypted or the typeof application is unknown to the SAN MAC layer LMAC. Moreover, theinterpretation of user data by communications interfaces may not beacceptable to users. Therefore, additionally or alternatively astatistical model may be stored at the SAN 6 and used to predict demandby the MAN's 2; optionally, the statistical model may be modified bymonitoring the traffic flow on individual duplex connections over theLESP and MESP channels and deducing statistical patterns. For example,it may be detected that a sequence of short data packets with a constantlength and interval transmitted to a service connection on the MAN 2 isusually followed by a high flow of data transmitted by the MAN 2 fromthat service connection. The statistical model is then updated so that,every time the same sequence of data packets is subsequently detected,additional capacity is allocated to the MAN 2 in the from-mobiledirection, if available. This reverse data flow prediction reduces theamount of queue status signalling that need to be transmitted by the MAN2.

The above embodiments have been described with reference to certainInmarsat™ systems purely by way of example and aspects of the presentinvention are not limited thereto. Instead, aspects of the presentinvention may be applied to terrestrial wireless networks, particularlythose that support contention-based access. The above embodiments areillustrated with reference to an architecture in which multiple mobileterminals access a network via a single access point (the SAN) via asatellite which acts only as a repeater. However, aspects of the presentinvention are also applicable to satellite networks in which one or moresatellites perform resource management and/or formatting functions.Furthermore, it is not essential that the mobile terminals receiveresource allocation signals from the same node with which the allocatedresources are used to communicate.

While the apparatus of the specific embodiments has been described interms of functional blocks, these blocks do not necessarily correspondto discrete hardware or software objects. As is well known, mostbaseband functions may in practice be performed by suitably programmedDSP's or general purpose processors and the software may be optimisedfor speed rather than structure.

1-12. (canceled)
 13. A method of transmission in a contention-basedaccess channel by a wireless transceiver, comprising: a) transmitting aburst in said channel; b) detecting whether said burst has collided withanother burst in said channel; and, if a collision is detected at saiddetecting step, waiting for a period determined according to a repeatparameter before repeating steps a) and b), wherein said repeatparameter is received by said transceiver.
 14. A method as claimed inclaim 13, wherein said period is randomly or pseudo-randomly selectedfrom a range indicated by said repeat parameter.
 15. A method as claimedin claim 13, wherein said repeat parameter includes a increment by whichsaid range is increased after each repetition of steps a) and b). 16-17.(canceled)
 18. A method as claimed in claim 26, including detecting thecontent of said monitored data, wherein the demand for capacity ispredicted according to said content.
 19. A method of allocatingfrequency channels to a plurality of wireless transceivers, comprising:transmitting to each of said transceivers a forward frequency channelallocation signal indicating an allocation of one or more forwardfrequency channels which that transceiver is to receive; andtransmitting to each of said transceivers, in at least one said forwardfrequency channels assigned to that transceiver, a respective returnchannel allocation signal indicating an allocation of one or more returnfrequency channels in which that transceiver may transmit; wherein, foreach forward frequency channel, a set of preferred return frequencychannels is stored, such that for each of said transceivers to which aspecified one of said forward frequency channels is allocated, theallocated one or more return frequency channels is preferentiallyselected from said corresponding set of preferred return frequencychannels.
 20. A method of allocating contention-based capacity to aplurality of wireless transceivers, comprising: transmitting to saidtransceivers a first contention-based capacity allocation signalindicating a first channel capacity assigned for contention-based accessto said transceivers; receiving in said first channel capacity,transmissions from said transceivers; detecting a level of usage by saidtransmissions of said first channel capacity; determining, according tosaid level and said first channel capacity, a second channel capacityassigned for contention-based access to said transceivers; andtransmitting a second contention-based capacity allocation signal,indicating said second channel capacity, to said transceivers. 21-25.(canceled)
 26. A method of controlling transmission by a wirelesstransceiver in a channel shared with transmission by other transceivers,comprising: monitoring data transmitted to said transceiver; predicting,on the basis of said monitoring step, a demand for capacity in saidchannel by said transceiver, and transmitting to said transceiver anallocation signal indicating an allocation in said channel determinedaccording to said predicted demand.
 27. A method as claimed in claim 26,including generating a statistical model based on previous traffic flowto and from wireless transceivers, wherein the demand for capacity ispredicted according to said statistical model.